Column grid array substrate attachment with heat sink stress relief

ABSTRACT

Structure and method for reinforcing a solder column grid array attachment of a ceramic or the like substrate to a printed circuit board, the reinforcement providing support for a heat sink which is bonded or affixed by pressure to a structural element of the substrate. In one form, the invention involves the concurrent formation of materially larger solder columns along the perimeter of the substrate in conjunction with the array of thin electrically interconnecting solder columns on the substrate. The reinforcing and electrical signal columns are thereafter aligned and attached by solder reflow to a corresponding pattern of pads on the printed circuit board. The heat sink is thermally connected to a structural element of the substrate by bonding or mechanical compression. Stresses in the solder columns caused by heat sink compressive forces or vibration induced flexing are materially decreased without adding complex or unique manufacturing operations.

FIELD OF THE INVENTION

The present invention is directed to column array connections attachingmodules or chips, generally substrates, with low coefficients of thermalexpansion to printed circuit boards having materially highercoefficients of thermal expansion. More particularly, the presentinvention is directed to structural refinements suitable to reinforcethe substrates for applications requiring heat sinks or the like.

BACKGROUND OF THE INVENTION

Integrated circuit packages and their applications have undergonetremendous change in evolving to what is presently considered acontemporary design. Integrated circuit die (chip) sizes havingincreased dramatically, as have their operational clock rates. At thesame time both the active and passive integrated circuit devicedimensions have decreases. The circuit functions available from eachintegrated circuit die are now materially greater. As a consequence ofsuch trends, integrated circuit packages require greater pin-out countsand higher power dissipation capabilities.

Numerous of the performance objectives were satisfied with ball gridarray technology, including both the flip-chip and ceramic packagevariants. The power dissipation problem was addressed through the use ofminiature heat sinks which attach directly to the flip-chip or ceramicpackage.

Simulation and testing of ball grid array type solder attachmentsinvolving silicon die or ceramic packages and underlying FR4 or the likefiberglass printed circuit boards has uncovered a susceptibility tostress failures. The failures occur with thermal cycling and areattributable to the materially different coefficients of thermalexpansion. The stresses experienced by the ball grid array solderconnections are aggravated with package size and with forces introducedby bonded or compressively affixed heat sinks. Moreover, the number ofthermal stress cycles, and associated fatigue failure rates, haveincreased materially with the introduction of die power managementtechniques which frequently cycle the die between sleep and fulloperation modes.

A very new connection technology capable of managing the strain causedby the mismatch in coefficients of thermal expansion involves the use ofsolder columns, rather than solder balls, to define the electricalconnections between the ceramic or die and the printed circuit board.Typical columns have an aspect ratio of approximately 9:2 and a nominaldiameter of 0.020 inches. The solder columns are formed from highmelting temperature solder using a nominal 90/10 alloy of lead to tin.The columns are first bonded to the ceramic or die, and thereafterattached to the printed circuit board using conventional low meltingtemperature solder paste reflow techniques.

As the power dissipation of flip-chip and ceramic packaged integratedcircuits have increased, now often exceeding 50 watts, the heat sink hasbecome critical necessity. Whether the heat sink is attached to thesubstrate by mechanical clamping, or by bonding, or the combination, theshock, vibration and pressure effects of heavy heat sinks are more thancolumn array solder connections alone can support.

One approach to reinforcing the solder column connections of ceramicpackage substrates involves the placement of kovar or cu-sil pins in thecorners of the ceramic packages to maintain the position of the ceramicpackage in relation to the printed circuit board in the presence of theheat sink vibrations and compressive forces. Such pins are attached tothe ceramic substrate by brazing. The pins are then positioned intoholes in the printed circuit board. Unfortunately, the use of such pinsresults in numerous additional and unique manufacturing steps.Furthermore, their use is effectively limited to ceramic packages, notfor flip-chip die attachments.

In view of the foregoing, there exists a need for both structures andmethods by which integrated circuits, whether in flip chip die orceramic packages, can be attached through a solder column grid arrayadequately to support a heat sink.

SUMMARY OF THE INVENTION

The present invention is practiced in the context of a system forconnecting a substrate having a low coefficient of thermal expansion toa printed circuit board having a materially higher coefficient ofthermal expansion using an array of solder columns and reflow bonding.In that context, the invention supports the substrate to permiteffective heat sink contact with a structural element of the substrate,comprising an array of high melting temperature solder columns of firstcross-sectional area attached to an array of electrically transmittingpads on the substrate, a set of high melting temperature solder columnsof second cross-sectional area, the second cross-sectional areaexceeding the first by a factor of 5 or greater, attached to pads atperimeter locations of the substrate, a plurality of connections betweenfirst and second cross-sectional area solder columns and respective padson the printed circuit board using reflowed low temperature solder, anda heat sink thermally contacting a structural element of the substrateon a side opposite the solder column attachments.

In another form, the invention is directed to a process practiced in thecontext of a system for connecting a substrate having a low coefficientof thermal expansion to a printed circuit board having a materiallyhigher coefficient of thermal expansion using an array of solder columnsand reflow bonding, the method providing supports for the substrate topermit effective heat sink contact with a structural element of thesubstrate, comprising the steps of attaching an array of high meltingtemperature solder columns of first cross-sectional area to an array ofelectrically transmitting pads on the substrate, attaching a set of highmelting temperature solder columns of second cross-sectional area, thesecond cross-sectional area exceeding the first by a factor of five orgreater, to pads at perimeter locations of the substrate, reflowing lowtemperature solder to connect first and second cross-sectional areasolder columns to respective pads on the printed circuit board, andconnecting by thermal contact a heat sink to a structural element of thesubstrate on the side opposite the solder column attachments.

In a particularized form, the invention extends and refines the basicpractice of using an array of solder columns to attach a substrate,whether that substrate is an integrated circuit die directly or a diemounted within a ceramic package, to a printed circuit board employingsolder paste and conventional reflow techniques. Relatively lower aspectratio solder columns are formed in the corners or other peripheralregions of the substrate, in conjunction with the formation of the thinsolder column electrical interconnect array, using corresponding highmelting temperature solder. The conventional and reinforcing columns arebonded to the substrate simultaneously using either a wire columnattachment process or a cast column attachment process.

The substrate and attached columns are aligned with the pads on theprinted circuit board using conventional fixtures. Likewise, bonding ofthe substrate to the printed circuit board as to both the reinforcingcolumns and conventional electrical signal column array is accomplishedby reflowing solder paste deposited on the printed circuit board pads.Once the substrate is bonded to the printed circuit board, thereinforcing columns support the substrate and any attached heat sink.The reinforcing solder columns maintain spacing and structural integrityin presence of vibration and temperature variations.

These and other features of the invention will be more clearlyunderstood and appreciated upon considering the detailed embodimentsdescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a side view of a prior art substrate andheat sink attachment using pins for structural support.

FIG. 2 is a schematic illustrating the connection of a substrate to aprinted circuit board using reinforcing solder columns according to thepresent invention.

FIG. 3 is a schematic showing a substrate attached with reinforcingcolumns to a printed circuit board and having a heat sink thermallycontacting a structural element of the substrate.

FIG. 4 schematically depicts process steps by which the invention may bepracticed in two variants.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates the prior art relating to thestructural support for a substrate connected by a solder column array toa printed circuit board when configured with a heat sink. As illustratedin the figure, ceramic substrate 1 has bonded thereto integrated circuitchip 2, chip 2 representing an individual integrated circuit die or acapped integrated circuit device. Electrically conductive pads 3 and 4,respectively formed on substrate 1 and FR4 type fiber glass printedcircuit board 6, define the pattern of the solder column array forelectrical input/output connections between substrate 1 and printedcircuit board 6. The connections between respective pads 3 and 4 areaccomplished using solder columns 7.

Solder column 7 are preferably formed from a 90/10 alloy of lead/tinsolder, namely, high melting temperature solder, with a nominal heightin the range of 0.08 inches and a nominal diameter of 0.02 inches.Solder columns 7 are attached to pads 3 of substrate 1 using either oftwo techniques. One process involves a high temperature reflow of the90/10 solder columns, with the columns mounted as an array in a boatwhich is affixed to the substrate. An alternate technique involves theuse of a low temperature solder paste deposited onto pads 3, followed bya low temperature reflow of the paste. The lower melting temperaturesolder, nominally 37/63 alloy lead/tin bonds an array of 90/10 soldercolumns which are mounted in a supporting boat to the substrate pads.Both techniques have been in commercial use.

The materially different coefficients of thermal expansion exhibited bysubstrate 1 and printed circuit board 6 are within the flexingcapabilities of solder columns 7 given the relative aspect ratio of thecolumns. Unfortunately, that aspect ratio makes the columns vulnerableto compressive and/or lateral forces, such as those created by spring 8and any movement of heat sink 9. Since the thermal resistance betweenheat sink 9 and chip 2 is inversely related to the clamping pressure,the structural limitations of solder columns 7 also constrain theability to effectively cool chip 2 when heat transfer is initiated byspring 8.

The solution developed heretofore, was to allow greater clampingpressure by using corner pins 11. Pins 11 are composed of kovar orcu-sil, exhibit a high compressive strength, and have minimal creep inrelation to solder columns 7. Pins 11 are bonded to substrate 1 bybrazing and extend into holes 12 drilled through printed circuit board6. Unfortunately, the use of such pins to support substrate 1 requiresthe additional manufacturing steps to braze attach pins 11, drill holes12 in printed circuit board 6, and to align pins 11 with holes 12 whenmounting substrate 1. The rigidity of the elements involved mandatesvery tight tolerances for the location of pins 11 and holes 12.

The present invention eliminates the additional manufacturing steps andtolerance requirements by extending the basic column attachment processto further support substrate 1, especially in the presence of heat sink9. The invention is applicable to situations in which substrate 1 is theceramic base of integrated circuit die 2, whether die 2 is capped orexposed. For a flip-chip application, die 2 is bonded to substrate 1using solder bumps. Though heat sink 9 in FIG. 1 is shown compressivelyaffixed to a structural element, chip 2 of the substrate using spring 8,the invention is also applicable to situations in which the heat sink isbonded directly to a structural element of the substrate, or iscompressively connected to such structural element using a mechanicallyadjustable clamp. Key to the invention is the ability to materiallyincrease the structural integrity of the interface between substrate 1and printed circuit board 6 for shock and vibration environments withinthe framework of conventional solder column fabrication practices.

FIGS. 2 and 3 illustrate the aspects of the present invention inconnecting substrate 1 to printed circuit board 6. As shown in FIG. 2,pads 3 of substrate 1 each have a solder column 7 attached by solderreflow. However, in keeping with the present invention, substrate 1 alsoincludes pads 13 situated along the perimeter of substrate 1, typicallyin the corners of the substrate, which have attached thereto individualsolder columns 14. Columns 14 are composed of high melting temperaturesolder, for example the earlier noted 90/10 alloy, and are attached whenelectrical signal columns 7 are bonded to pads 3 of substrate 1. Soldercolumns 14 along the perimeter of substrate 1 provide the structuralsupport while the array of solder columns 7 provide the high densityelectrical connections. Preferably, columns 14 are in the range of 0.08inches or greater in diameter while matching the height of columns 7.

The connection of columns 7 and 14 to corresponding pads 4 and 16 onprinted circuit board 6 is accomplished by reflow of low meltingtemperature solder paste 17 after the columns 7 and 14 are positioned insolder paste. Solder paste 17 is composed of flux and nominal 37/63composition lead/tin solder having a melting temperature ofapproximately 180 degrees C.

FIG. 3 schematically illustrates the structural arrangement after heatsink 9 is mounted. Heat sink 9 can be held in place using spring 8extending through holes 18 in printed circuit board 6, or by directbonding between heat sink 9 and chip 2 using thermal epoxy material.Support and spacing solder columns 14 are bonded to printed circuitboard pads 16 by reflowed solder 19 while the array of columns 7 areelectrically connected to pads 4 on printed circuit board 6 withreflowed solder 21.

Finite element simulations of a ceramic substrate with an aluminumcapped chip and heat sink, attached to an FR4 type printed circuit boardand subjected to various shock and vibration conditions have confirmedthat the stresses imposed upon the thin solder columns are materiallyreduced when supporting solder columns are placed along the perimetersof the substrate. For purposes of the simulation various dimensions ofsupporting solder columns 14 were considered. As the support columnsincreased in cross-sectional area, the stresses in the thin soldercolumns decreased. For example, with solder columns having a height of0.09 inches, and electrically transmitting solder columns 7 having anominal diameter of 0.02 inches, the use of supporting solder columnshaving a cross-sectional area of five times the electricallytransmitting solder columns caused the stress in the thin solder columnsto decrease by approximately 50 percent.

What makes the present invention particularly valuable is the fact thatthe benefits are obtained without the added complexity of brazing pins,drilling additional holes in the printed circuit board, or aligning thepins and the holes during manufacture.

FIG. 4 schematically illustrates alternate operations for attachingsolder columns to the substrate. One involves a reflow of lowtemperature solder paste, while the other consists of a direct reflow ofthe high temperature solder in each column. The descriptions in thevarious blocks are relatively self explanatory. In general, when lowtemperature solder is used to attach the solder columns to thesubstrate, including both the thin electrically signal solder columnsand the structural supporting solder columns, all the columns are loadedin a common boat, solder paste is deposited on the substrate pads, andthe solder paste is reflowed at 210-220 degrees C. On the other hand,where the solder columns themselves are reflowed as an aspect of theattachment to the substrate, the reflow is performed without solderpaste but at the materially higher temperature of approximately 350-360degrees C. Therefore, both types of solder columns are attachedsimultaneously both to the substrate and to the printed circuit board.

It should be understood that structural support solder columns 14 do notneed to be circular in cross-section. The invention fully contemplatessquare or other cross-sections, as the substrate area and loadconditions dictate. Similarly, as noted generally hereinbefore, chip 2may be an actual integrated circuit die or the cap of an integratedcircuit die. In either case, it constitutes a structural element ofsubstrate 1 for purposes of transferring heat to heat sink 9. Lastly, itshould also be recognized that columns 14 are fully capable oftransmitting electrical power or functional input/output signals, andare likely to be used for such as the pin-out needs of integratedcircuit devices increase.

Although the invention has been described in the context of a substrate,typically ceramic, onto which an integrated circuit die is attached, thefundamental concepts are fully applicable to a situation in which thesubstrate itself is a flip-chip device. In that context the columns arebonded directly to the flip-chip die pads, and the heat sink isthermally connected to the back side of the flip-chip die. Such aflip-chip device can be connected directly to an organic printed circuitboard, a ceramic carrier, or other die wiring package.

Though the invention has been described and illustrated by way ofspecific embodiments, it is intended that the apparatus and methods inthe appended claims encompass changes within the scope of the inventionis broadly disclosed.

What is claimed is:
 1. In a system for connecting a substrate having alow coefficient of thermal expansion to a printed circuit board having amaterially higher coefficient of thermal expansion using an array ofsolder columns and reflow bonding, a supporting structure with effectiveheat sink coupling to the substrate, comprising: an array of highmelting temperature solder columns of first cross-sectional areaattached to an array of electrically transmitting pads on a first sideof the substrate; a set of high melting temperature solder structuralsupport columns of second cross-sectional area, the secondcross-sectional area exceeding the first by a factor of five or greater,attached to pads at perimeter locations on the first side of thesubstrate; a plurality of connections between first and secondcross-sectional area solder columns and respectively located surfacemount pads on the printed circuit board using reflowed low meltingtemperature solder; and a heat sink thermally contacting a structuralelement on a second side, opposite the first side, of the substrate. 2.The apparatus recited in claim 1, wherein the thermal contact betweenthe heat sink and the structural element of the substrate is by bonding.3. The apparatus recited in claim 2, wherein high melting temperaturesolder is nominally 90/10 of lead/tin.
 4. The apparatus recited in claim3, wherein second cross-sectional area solder columns are connected totransmit electrical signals between the pads on the substrate and on theprinted circuit board.
 5. The apparatus recited in claim 4, wherein lowmelting temperature solder is nominally 37/63 of lead/tin.
 6. Theapparatus recited in claim 1, wherein the thermal contact between theheat sink and structural element of the substrate is by mechanicalcompression.
 7. The apparatus recited in claim 6, wherein the mechanicalcompression is caused by a spring acting between the heat sink and theprinted circuit board.
 8. The apparatus recited in claim 7, wherein highmelting temperature solder is nominally 90/10 of lead/tin.
 9. Theapparatus recited in claim 8, wherein second cross-sectional area soldercolumns are connected to transmit electrical signals between the pads onthe substrate and on the printed circuit board.
 10. The apparatusrecited in claim 9, wherein low melting temperature solder is nominally37/63 of lead/tin.
 11. In a system for connecting a substrate having alow coefficient of thermal expansion to a printed circuit board having amaterially higher coefficient of thermal expansion using an array ofsolder columns and reflow bonding, an apparatus which supports thesubstrate, comprising: an array of high melting temperature soldercolumns of first cross-sectional area attached to an array ofelectrically transmitting pads on a first side of the substrate; a setof high melting temperature solder structural support columns of secondcross-sectional area, the second cross-sectional area exceeding thefirst by a factor of five or greater, attached to pads at perimeterlocations on the first side of the substrate; and a plurality ofconnections between first and second cross-sectional area solder columnsand respectively located surface mount pads on the printed circuit boardusing reflowed low melting temperature solder.
 12. The apparatus recitedin claim 11, wherein high melting temperature solder is nominally 90/10of lead/tin.
 13. The apparatus recited in claim 12, wherein secondcross-sectional area solder columns are connected to transmit electricalsignals between the pads on the substrate and on the printed circuitboard.
 14. The apparatus recited in claim 13, wherein low meltingtemperature solder is nominally 37/63 of lead/tin.